The present invention relates to an information communication device, such as a non-contact tag or non-contact IC card, that processes a signal by obtaining electric power from an electric wave transmitted from an antenna.
Conventionally, non-contact tags have been in use that transmit the information stored in themselves by acquiring electric power from an electric wave transmitted from somewhere else. Non-contact tags are used, for example, with the lift facilities at a skiing ground, with the ticket examination equipment at a railway station, and for sorting of baggage.
Such a non-contact tag has a transmitter/receiver unit incorporated therein, but not a power source such as a battery. A non-contact tag obtains electric power from an electric wave (a high-frequency signal) it receives. Thus, a non-contact tag, despite incorporating no battery, permits exchange of information for a long period of time. Moreover, a non-contact tag achieves exchange of information by an electric wave or by magnetic induction, and thus offers the advantage of exchanging information on a non-contact basis.
On the other hand, for the purpose of recording or, inversely, offering information, IC cards are already in practical use that permit exchange of information with a reader/writer on a non-contact basis (thus by an electric wave or by magnetic induction). Such an IC card, too, obtains electric power from an electric wave it receives or through magnetic induction.
In FIG. 9, reference numeral 90 represents a reader/writer, with only a transmission antenna (coil LS) provided therein shown as a constituent component thereof. On the other hand, reference numeral 91 represents an IC card, reference numeral 92 represents a tuning circuit thereof, and reference numerals 93 and 94 represent a rectifying diode and a smoothing capacitor, respectively. The tuning circuit 92 is composed of a reception antenna (coil LR) and a tuning capacitor C0. An electric wave transmitted from the reader/writer 90 is received by the tuning circuit. The thus received high-frequency signal is rectified by the rectifying diode 93 and is then smoothed by the smoothing capacitor 94 so as to be used as a power source by the IC card 91.
The intensity (amplitude level) of the high-frequency signal received by and output from the tuning circuit 92 varies according to the distance between the reader/writer 90 and the IC card 91; specifically, the shorter the distance, the more intense the signal, and, the longer the distance, the less intense the signal. Thus, the reproduction efficiency with which electric power is produced by rectifying the received high-frequency signal also varies according to the distance between the reader/writer 90 and the IC card 91.
As a result, quite inconveniently, the range of electric power that needs to be fed to the IC card to ensure its normal operation (in other words, the range of non-contact distance) is rather limited (narrow). Such a limited range of acceptable input voltages makes the IC card unusable in applications in which the input voltage tends to vary greatly. For this reason, it is customary to limit the specifications of IC cards in accordance with their applications (i.e. manufacture separate models for short-distance and long-distance applications). This, however, requires extra cost and causes inconvenience.
Moreover, in this conventional example, a diode is used as a rectifying means, and this diode cannot be used simultaneously as a tuning capacitor. Thus, it is inevitable to provide a capacitor separately in the tuning circuit or secure a capacitance by making the tuning coil larger.
An object of the present invention is to provide a communication device that accepts a larger range of input voltages.
Another object of the present invention is to provide a communication device in which a rectifying means is used to secure a capacitance for a tuning circuit.
To achieve the above objects, according to the present invention, in a communication device that produces electric power by rectifying a received high-frequency signal, a MOS transistor is connected to an antenna so that the received signal is rectified by a parasitic diode of the MOS transistor. In this case, the capacitance accompanying the MOS transistor influences the input tuning frequency.
The communication device may be provided with, as the above-mentioned MOS transistor, one or more MOS transistors, and in addition have control means for controlling the on/off states of the transistors. This control means controls the MOS transistors in such a way as to change the tuning frequency in the direction in which the rectified output becomes equal to a predetermined level. The communication device is built, for example, as an IC card that receives the signal from a reader/writer on a non-contact basis.
Thus, according to the present invention, a communication device produces electric power by rectifying a received high-frequency signal. The received signal is rectified by a parasitic diode of a MOS transistor that is connected to an antenna. The capacitance accompanying the MOS transistor serves as an input tuning capacitance. Switching the on/off state of the transistor causes the capacitance added to that of an input tuning capacitor to change; accordingly, the input tuning frequency changes and the relation between the input tuning frequency and the frequency of the received high-frequency signal changes. As a result, the amplitude of the received high-frequency signal changes and thus the obtained power also changes.